Rifled ammunition system

ABSTRACT

Gun and ammunition solutions which permit the use of High Explosive (HE) rounds and Armor Piercing Fin Stabilized Discarding Sabot (APFSDS) rounds in one weapon. A first system has a gun barrel chamber sized to receive the outer cases of both APFSDS and HE rounds. The chamber has a smooth rear section which abuts a front section having internal helical rifling. A HE round has an outer case with a length that is the same as the length of the smooth rear section of the barrel. A projectile held by the case has a rotating band on a rear end configured to engage the internal helical rifling in the chamber. A second system has a conventional smooth barrel and utilizes a HE round with an outer case having internal rifling in a forward section. A projectile held by the outer case has a circular band mounted to rotate about a rear end thereof to engage the internal rifling.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 16/782,810, filed Feb. 5, 2020, which is a continuation of U.S. application Ser. No. 16/290,716, filed Mar. 1, 2019, which claims priority under 35 USC § 119 to U.S. Provisional Application Ser. No. 62/637,237, filed Mar. 1, 2018, the entire disclosure of which is expressly incorporated herein.

FIELD OF THE INVENTION

The present invention is directed to guns and ammunition and, more particularly, to solutions enabling use of both High Explosive (HE) and Armor Piercing Fin Stabilized Discarding Sabot (APFSDS) rounds in the same gun without loss of effectiveness.

BACKGROUND OF THE INVENTION

Armor-piercing discarding sabot (APDS) was initially the main design of the kinetic energy (KE) penetrator for armor-piercing rounds. The logical progression was to make the penetrator longer and thinner to concentrate the kinetic energy in a smaller area. However, a long, thin rod is aerodynamically unstable; it tends to tumble in flight and is less accurate. Traditionally, rounds were given stability in flight from the rifling of the gun barrel, which imparts a spin to the round. Rifling is the helical groove pattern that is machined into the internal (bore) surface of a gun's barrel, for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting. This spin serves to gyroscopically stabilize the projectile by conservation of angular momentum, improving its aerodynamic stability and accuracy over smoothbore designs. Up to a certain limit, this is effective, but once the projectile's length is more than six or seven times its diameter, rifling becomes less effective. Adding fins like the fletching of an arrow to the base gives the round stability, leading to the Armor Piercing Fin Stabilized Discarding Sabot rounds (APFSDS). The spin from standard rifling decreases the performance of these rounds (rifling diverts some of the linear kinetic energy to rotational kinetic energy, thus decreasing the round's velocity and impact energy), and very high rotation on a fin-stabilized projectile can dramatically increase aerodynamic drag, further reducing impact velocity.

For these reasons, APFSDS projectiles are generally fired from smoothbore guns, a practice that has been taken up for tank guns by China, India, Israel, Italy, Japan, France, Germany, Turkey, Russia, and the United States. Nevertheless, in the early development of APFSDS ammunition, existing rifled barrel cannons were used, (and are still in use), such as the M68-105mm cannon mounted on the M60A3 main battle tank. To reduce the spin rate when using a rifled barrel, a “slip obturator” (slip obturation ring), is incorporated that allows the high pressure propellant gasses to seal, yet not transfer the total spin rate of the rifling into the projectile. The projectile still exits the barrel with some residual spinning, but at an acceptably low rate.

A problem has arisen in the development of a modern family of ammunition tied to the different spin requirements of High Explosive (HE) rounds and Fin Stabilized Armor Piercing Discarding Sabot (APFSDS) rounds. HE rounds need to be spun to a high rotational rate to satisfactorily stabilize the round and to act as a function in the fuse safety mechanism. They therefore need a normal rifled barrel. On the other hand, an APFSDS round needs to be spun very slowly or not at all and therefore needs an unrifled or smooth bore barrel.

Many ideas have been proposed and tried to mitigate these mutually competing requirements, from slipping rotating bands for the APFS rounds to fin stabilizing the HE rounds. None of those approaches have proven to be satisfactory. They all have been bad compromises. This is particularly true of the fin stabilized HE where having satisfactory fuse safety is difficult without the use of the spin function.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved gun and ammunition solutions which permit the use of High Explosive (HE) rounds and the Armor Piercing Fin Stabilized Discarding Sabot rounds (APFSDS) in one weapon using a smooth bore barrel. Alternatively, the chamber of the barrel is modified with partial rifling to enable use with both types of rounds.

The present application contemplates a first firearms system comprising a gun barrel for use with both armor-piercing and high explosive rounds. The gun barrel has a chamber adjacent a breach with a diameter sized to receive the outer cases of both armor-piercing and high explosive rounds. The chamber has a smooth rear section which abuts a front section having internal helical rifling. A high explosive round for use with the gun barrel of the first system has an outer case with a length that is approximately the same as the length of the smooth rear section of the barrel. The high explosive round further includes a projectile held by the case with a rotating band on a rear end thereof configured to engage the internal helical rifling in the front section of the chamber. An armor-piercing round for use with the gun barrel of the first system has an outer case with a length that is approximately the same as the chamber length and a sealing ring at a forward end sized to engage a smooth bore portion of the barrel forward of the chamber.

In the gun barrel in the first firearms system, the front section desirably has a length of about half of the chamber length, and the chamber length may be between 8-15 inches, in some cases 12 or 13 inches. Preferably, the front section of the gun barrel is straight and the rear section is tapered.

The present application contemplates a second firearms system comprising a high explosive round including an outer case containing a propellant charge in an inner cavity terminating in a rearward closed end and having internal rifling in a forward section. The outer case has a longitudinal length along an axis, wherein the internal rifling is disposed only in the forward section. A projectile is held by the outer case in a forward position of the cavity, the projectile having a circular band mounted to rotate about a rear end of the projectile. The band is positioned at a rear end of the forward section with internal rifling. The system comprises a gun having a smooth bore barrel with a chamber having a length the same as the longitudinal length of the outer case of the high explosive round. The second system also incorporates an armor-piercing round having an outer case with a length the same as the longitudinal length of the outer case of the high explosive round and a sealing ring at a forward end sized to engage the barrel forward of the chamber. The same gun may be used to fire either the high explosive round or the armor-piercing round, which is preferably an Armor Piercing Fin Stabilized Discarding Sabot round (APFSDS).

In the high explosive round of the second firearms system, the band preferably also has external rifling at the same angle as an angle of the internal rifling. The band may be formed of plastic or metal. The longitudinal length of the outer case of the high explosive round of the second firearms system may be between about 8-15 inches, and the longitudinal length of the forward section is preferably about half the longitudinal length of the outer case. In one embodiment, the band has a length of between 5-10% of the longitudinal length of the outer case.

A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become appreciated as the same become better understood with reference to the specification, claims, and appended drawings wherein:

FIG. 1 is a sectional view of an APFSDS round modified in accordance with the present application to function with the gun of FIG. 2;

FIG. 2 is a sectional view of a duplex chamber region of a gun barrel in accordance with the present application, and FIG. 2A is a sectional view of a portion of an inner wall of the barrel showing raised rifling;

FIG. 3 is a sectional view of an HE round modified in accordance with the present application to function with the gun of FIG. 2;

FIG. 4 is a sectional view of an APFSDS round modified in accordance with the present application to function with the gun of FIG. 5;

FIG. 5 is a sectional view of a smooth chamber region of a gun barrel;

FIG. 6 is a sectional view of an HE round modified in accordance with the present application to function with the gun of FIG. 5;

FIG. 7 is a side view of the HE round of FIG. 6;

FIG. 8 is a sectional view through a side wall of the case of the HE round of FIG. 6;

FIG. 9 is a longitudinal sectional view of a conventional shotgun shell with a slug instead of shot; and

FIG. 10 is a longitudinal sectional view of a shotgun shell with a slug in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present application is directed to improved gun and ammunition solutions which permit the use of High Explosive (HE) rounds and Armor Piercing Fin Stabilized Discarding Sabot (APFSDS) rounds in one weapon using a smooth bore barrel. Alternatively, the chamber of the barrel is modified with partial rifling to enable use with both types of rounds. Similar techniques are also used in an improved shotgun shell with a slug.

Configuration #1—Duplex Barrel & Chamber

One solution to enabling the use of both APFSDS and HE rounds in one gun is to modify the gun barrel to have a portion with rifling and modify the case of the HE round so that the projectile engages the rifling. An APFSDS round 14 is seen in FIG. 1 while an HE round 16 is shown in FIG. 3, both of which may be fired from a gun having the barrel 18 of FIG. 2. For the purpose of orientation, the rear ends of the rounds 14, 16 are to the right, while their forward ends are to the left. It should be understood that the right end of the barrel 18 is shown open but would typically be closed by the breach with firing pin. Both the APFSDS round 14 and HE round 16 are sized to fit closely within the tubular barrel 18, and both have a cavity in the case behind a projectile held by the case filled with a propellant charge (not shown).

In the configuration illustrated in FIGS. 1-3, two different length of cartridge cases are used. The APFSDS round in FIG. 1 uses a full-length case 22, and the HE round in FIG. 3 uses a half-length case 20. In the barrel shown in FIG. 2, the chamber is divided into two sections. A rear section 30 is smooth and slightly tapered like a normal chamber whereas a front section 32 is straight (parallel to the longitudinal axis) and rifled. The rifling in the front section 32 is much steeper than in a normal barrel; such as a 20° or more helix angle HA (e.g., 20-26°) in contrast to 7-10° helix angles in normal barrel rifling. FIG. 2A is a sectional view of a portion of an inner wall of the barrel showing the rifling as being raised as opposed to grooves, which may be formed by electro discharge machining (EDM). The barrel forward of the front section 32 has a smooth bore 34.

With the HE round and the half-length case of FIG. 3, a rotating band 24 on a rear end of the projectile 25 is clear to engage the rifling in the front part 32 of the chamber. The rotating band 24 may be made of plastic or metal, and is desirably smooth on its exterior surface. Conversely, for the APFSDS round which uses the long case 22, a front edge of the case extends past the rifling 32 and positions a sealing ring 26 of the round in the start of the smooth bore portion 34 of the barrel. Thus, no rotation is imparted to the APFSDS round. It should be noted that the sealing ring 26 has a forward portion that extends outward slightly from a rear portion held within the case 22 so that the case and sealing ring form a continuous outer cylindrical surface.

The chamber portion of the barrel made up of the rear section 30 and front section 32 preferably has a length L_(B) of between 8-15 inches, desirably 12 or 13 inches. The APFSDS round case 22 has a length L_(AP) of between 8-15 inches, desirably 12 or 13 inches, which is as long as or slightly longer than the chamber length L_(B). The length L_(R) of the rifled section 32 of the barrel is desirably about half of the chamber length L_(B). For instance, the length L_(R) may be 4-6 inches while the length L_(B) is 8-12 inches, and specifically twice the length L_(R). Similarly, the length L_(C) of the HE round case 20 is also desirably about half of the chamber length L_(B), and is more preferably L_(B)-L_(R). This places the rotating band 24 just at the start of the rifled section 32 so as to impart maximum rotation momentum to the projectile.

Rifling may be described by its helix angle HA or by the twist rate, which indicates the distance the rifling takes to complete one full revolution, such as 1 turn in 48 inches (122 cm). A shorter distance indicates a “faster” twist, meaning that for a given velocity the projectile will be rotating at a higher spin rate. The combination of length, weight and shape of a projectile determines the twist rate needed to stabilize it—barrels intended for short, large-diameter projectiles like spherical lead balls require a very low twist rate, such as 1 turn in 48 inches (122 cm). Barrels intended for long, small-diameter bullets, use faster twist rates.

The helix angle HA of the rifling in the front half section 32 of the barrel chamber may be 20° or more (e.g., 20-26°). The corresponding twist rate depends on the barrel ID but is faster than conventional twist rates, such as 1 turn in 4-5 inches, because of the steeper helix angle HA.

Previous research in the area of extraction forces and the influence of case design and sticking locations leads the inventor to believe that while the long case extraction forces would be higher than without the rifling, they would not be excessive or limiting. One means of limiting those forces would be to relieve the rear face 27 of the cartridge case where it bears against the bolt face. Namely, a small circular step 28 is provided at the outer diameter of the rear face 27 that reduces the diameter of the rear face 27 inward to the same diameter as the inside diameter of the case, or to the diameter of the adjacent groove 29 just forward of the rear flange of the case. This would allow the case to more easily rotate against the bolt face as it is extracted. Another solution is to use brass as the case material instead of steel for its superior extraction characteristics.

Configuration #2—Spun/Non-Spin Ammunition

Rather than modifying the gun barrel, the HE round may be modified, as seen by the APFSDS round 50, gun barrel 60 and HE round 70 in FIGS. 4-6. As with FIGS. 1-3, APFSDS round 50 and HE round 70 are adapted to both be shot from the gun barrel 60, which has a smooth bore 62 and an inward step 64 delineating a larger diameter rear portion from a smaller diameter front portion. The APFSDS round 50 in FIG. 4 has a standard smooth case 52, and a sealing band 54 is disposed at the mouth (forward end) of the case. Here it should be noted that the sealing band 54 has a continuous outer cylindrical surface that both fits and is held within the case 52 and also slides easily through the smooth bore 62 forward of the inward step 64.

The solution for enabling HE rounds to be fired from a smooth bore 62 barrel is to incorporate the rifling 40 into the forward section of a cartridge case 42 itself, as illustrated in FIGS. 6 and 7. A projectile 44 is held by the outer case 42 in a forward position of an internal cavity 45 otherwise filled with a propellant charge. The rifling 40 in the forward section of a cartridge case 42 may be done by roll forming helical rifling grooves 40 (FIG. 7) into the wall of the case itself. (In this sense, grooves are formed from the outside projecting inward so that the rifling 40 comprises raised helical ribs extending inward into the case 42.) Since the length of the rifling 40 would be quite short, with the rest of the case being smooth 46, the rifling helix angle HA would be considerably steeper than the rifling in a full-length barrel. So, for example, the rifling twist rate may be as fast as 1 turn in 4-5 inches (or a 20° or more helix angle, e.g., 20-26°), in contrast to 7-10° helix angles in normal barrel rifling.

FIG. 8 shows some exemplary radial dimensions of the case 42 and rifling grooves 40 thereon. In one embodiment, the total length L_(HE) of the case 42 of the HE round 70 is between about 8-15 inches, desirably 12 or 13 inches, which is as long as the large diameter portion of the smooth bore 62. The forward portion of the case 42 having rifling grooves 40 may be about half the length L_(HE), or between about 4-7.5 inches, desirably about 6 inches.

To prepare the HE projectile for use in this type of rifled case it is anticipated that the projectile would be made with a pre-rifled band 48, preferably of a suitable plastic, configured to rotate about a rear end of the projectile. Preferably, the length L_(B) of the band 48 is between 5-10% of the total case length L_(HE), more preferably 5%. In absolute terms, the length L_(B) of the band 48 may be between 0.5-1.0 inches. If the band 48 is plastic it is formed longer, while a metallic band would be shorter. The band has external rifling at the same angle as an angle of the internal rifling and is engaged therewith so that as soon as the round is fired the band 48 starts spinning as the projectile moves through the case 42.

When the projectile 44 leaves the rifled case 42 and enters the smooth bore barrel 62 past the inward step 64, this band 48 would then be swaged down to the bourrelet diameter of the projectile. The bourrelet is the portion of an elongated artillery projectile having a very slightly smaller diameter than the interior diameter of the barrel through which that projectile is to be propelled. This is done to enhance the sealing of the projectile against gas blow by, and to reduce to an absolute minimum the yaw of the projectile in the bore of the barrel. This reduced yaw is essential for the attainment of superior accuracy and dispersion.

The rifled case 42 would only be used on the HE round. Grooves have been formed in cases before. They have been used to crimp projectiles in the case or as locating grooves for belt links and conveyor system elements. However, in those instances the grooves have been circumferential not longitudinal or helical.

The rifling grooves 40 are shown formed in the case 42 in FIGS. 7 & 8, and show on both the inside and the outside of the case. FIG. 8 shows a radial sectional view of one portion of the wall of the case 42 wherein the rifling projects inward. The combination of the inwardly-projecting rifling defines a diameter D_(P) which is the same as the diameter of the projectile 44. Portions of the wall of the case 42 in between the rifling 40 are left unchanged, and define a diameter D_(B) which is the same as the maximum outer diameter of the rotating band 48. That is, the rotating band 48 has rifled grooves that engage the inwardly-projecting rifling 40. The diameter D_(P) of the projectile varies. The wall thickness of the case 42 also may vary, and in one embodiment is 0.025 inches, and the extent to which the rifling grooves 40 project inward may be approximately twice that amount, or in the exemplary embodiment 0.050 inches.

Based on previous experience and using the 50 mm system as an example, there is about 6 inches of available length out of the 13 inches of total case length for rifling. This amounts to about three times the bore diameter. At that point in the projectile's travel it is moving at about 800 to 900 feet per second. Again, previous experience has shown that a helix angle HA of 20 to 25 degrees will give sufficient rotation to stabilize the projectile at the muzzle exit velocity of 3000+fps. During the projectile's travel through the smooth bore barrel 62, there will be some decay of the initial spin, but more than enough remaining to stabilize the bullet on exit.

The barrel 60 itself in FIG. 5 of course has a smooth bore based on the bourrelet diameter of the projectile not the rotating band diameter. This is also the diameter of the sealing ring 54 on the APFSDS Projectile.

An anticipated problem with the rifled case 42 is that the rifling grooves 40 that are rolled in the case might be blown back flat against the chamber wall during firing and not properly spin the bullet. Theoretically the chamber pressure required to do this is still behind the projectile while the rifling is being used. It is therefore protected until after it has performed its function. To counter this possibility, it might be necessary to fill the grooves on the outside of the case with a non-compressible plastic or similar material, as indicated at 72 in FIG. 8. It is however believed that if the grooves are properly designed and manufactured, they will withstand the stresses of firing.

Configuration #3—Shotgun Slug Ammunition

Another related use of the rifling concept is in the improvement of the effectiveness of single projectile ammunition for shotguns. This ammunition is commonly called “rifled slugs.” Shotguns are often used in hunting, especially in areas where the use of rifles are prohibited by wildlife regulations because of their long-range and the unintended consequences of missed shots. When hunting large game such as deer, slugs in the shotgun shells replace conventional shot, which tend to cause more damage than desirable. Although shotgun shells with slugs are effective, they are not very accurate beyond a limited range. Consequently, the present application contemplates an improved shotgun shell with a slug which increases accuracy and reduces dispersion over a longer range.

The most common way of stabilizing the projectile is to put “rifling” on the outside of the projectile with the hope that its interaction with the barrel will cause some rotation and stabilize the round. Slugs are generally very short and have a very low L/D so the spin requirement is also low and this inside out rifling works to an extent, but the dispersion and accuracy is usually very poor such that it is a very short-range system. In addition, variations in the bore and choke of the shotgun's barrel greatly effects the performance of this system, meaning that a gun designed for slugs might work reasonably well but not in a gun optimized for bird shot.

FIG. 9 is a longitudinal sectional view of a conventional shotgun shell 80 with a slug instead of shot. The cartridge case of a standard shotgun shell is usually made of plastic which is reinforced at the base with a thin brass outer shell. Further reinforcement is provided by heavy plastic inner plug to support the primer and to give strength to the case in the breech area. The shell 80 typically includes a flanged rear brass cup 82 having a primer cap 84 centered therein. A majority of the length of the shell 80 has a plastic outer casing 86 terminating at an open forward end. The contents of the shell 80 include a propellant charge 88 at a rear end, a so-called wad 90 to hold the propellant charge in place, and a slug 92 positioned adjacent the open forward end. In this example, the slug 92 has external rifling. The heavy plastic inner plug 94 is typically inserted in the rear end of the cavity to support the primer 84 and to give strength to the case in the breech area.

FIG. 10 is a longitudinal sectional view of a shotgun shell 100 with a slug in accordance with the present invention. The shell 100 includes a single, unitary outer case 102 having a flanged rear end 104 with a conventional rifle primer cap 106 centered therein. The outer case 102 is desirably formed of a metal such as brass, aluminum or steel. The case 102 has straight tubular walls 108 (i.e., parallel to a central axis) which extend from the rear end 104 to an open forward end 110. A quantity of a propellant charge 112 is provided in a rear portion of the cavity defined by the case 102. A projectile or slug 114 is positioned within the cavity of the case 102 directly against the propellant charge 112. In the illustrated embodiment, the slug 114 has a rear, preferably hemi-spherical cavity 116 also filled with the propellant charge. The illustrated slug 114 also has a rounded or bullet-shaped front end 118, and a generally cylindrical outer wall 120. The slug 114 is roughly the same as the prior art slug 92 in FIG. 9, and the amount of a propellant charge 112 is also about the same as in prior shells.

In the present invention, the metal cartridge case 102 is used in place of the plastic outer casing 86 from the standard shell, which would never be able to support rifling. Also, with the metal case 102, there is no need for the heavy plastic base plug 94 which gives more room for the propellent charge 112 and allows the slug 114 to be moved further back in the case so as to give more length for the rifling and engagement therewith by the slug upon firing.

A forward section of the inner wall of the case 102 has rifling 122, but is otherwise empty. In a preferred embodiment, the slug 114 is positioned to the rear of a midway point along the cavity of the case 102 such that the rifling 122 occupies a majority of the length of the case, preferably between about 50-60% thereof. To prevent debris from entering the open forward end 110, a cover 124 is secured thereover. The cover 124 may be formed a variety of materials, including paper or plastic, and typically simply wraps tightly around the outer wall of the case 102 for a short distance. Typical lengths of the shotgun case 102 is 2-3 inches, so the rifling would extend between 1-1.2 inches for a 2-inch shell, and 1.5-1.8 inches for a 3-inch case, with the corresponding range of dimensions in between.

The rifling 122 desirably has a twist rate that is relatively steep, such as 1 turn in 10 inches (or a 10-15° helix angle), so that the slug has sufficient spin when it reaches muzzle velocity. A shotgun barrel might have a twist rate of 1 turn every 48 inches. By using a rifled case, problems that arise with the use of externally rifled slugs in shotgun barrels design for shot shells is avoided. The internal case rifling is consistent and predicable. Additionally, since the muzzle exit rotational velocity is quite low due to the low L/D the rifling in the case has a very moderate twist angle. Also, the straight case sides and relatively long cases available in shotgun ammunition make those rounds ideal for this concept.

Upon reading this disclosure, those skilled in the art will appreciate that various changes and modifications may be made to the preferred embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept. 

What is claimed is:
 1. A firearm system for use with both armor-piercing and high explosive rounds, comprising: a chamber between a breach and a barrel, the chamber having a diameter sized to receive the outer cases of both armor-piercing and high explosive rounds, the chamber having a smooth rear section which abuts a front section having internal helical rifling.
 2. The system of claim 1, wherein the front section has a length of about half of the chamber length.
 3. The system of claim 2, wherein the chamber length is between 8-15 inches.
 4. The system of claim 2, wherein the front section is straight and the rear section is tapered.
 5. The system of claim 1, wherein the front section is straight and the rear section is tapered.
 6. The system of claim 1, further including a high explosive round having an outer case with a length that is approximately the same as a length of the smooth rear section of the chamber of the barrel, and a projectile held by the outer case with a rotating band on a rear end thereof configured to engage the internal helical rifling in the front section of the chamber.
 7. The system of claim 6, wherein the rotating band is formed of plastic and has a smooth outer surface.
 8. The system of claim 1, further including an armor-piercing round having an outer case with a length that is approximately the same as the chamber length and a sealing ring at a forward end positioned within and sized to engage a smooth bore portion of the barrel forward of the chamber.
 9. The system of claim 1, wherein the internal helical rifling has a helix angle of between 20-26°.
 10. The system of claim 1, wherein the internal helical rifling is formed by raised helical ribs extending inward within the chamber.
 11. A firearm system for use with both armor-piercing and high explosive rounds, comprising: a chamber between a breach and a barrel, the chamber having a diameter sized to receive the outer cases of both armor-piercing and high explosive rounds, the chamber having a smooth tapered rear section which abuts a straight front section having internal helical rifling having a helix angle of between 20-26°.
 12. The system of claim 11, wherein the front section has a length of about half of the chamber length.
 13. The system of claim 12, wherein the chamber length is between 8-15 inches.
 14. The system of claim 13, wherein the chamber length is 12 or 13 inches.
 15. The system of claim 11, wherein the chamber length is between 8-15 inches.
 16. The system of claim 11, further including a high explosive round having an outer case with a length that is approximately the same as a length of the smooth rear section of the chamber of the barrel, and a projectile held by the outer case with a rotating band on a rear end thereof configured to engage the internal helical rifling in the front section of the chamber.
 17. The system of claim 16, wherein the rotating band is formed of plastic and has a smooth outer surface.
 18. The system of claim 16, wherein the rotating band is formed of metal and has a smooth outer surface.
 19. The system of claim 11, further including an armor-piercing round having an outer case with a length that is approximately the same as the chamber length and a sealing ring at a forward end positioned within and sized to engage a smooth bore portion of the barrel forward of the chamber.
 20. The system of claim 11, wherein the internal helical rifling is formed by raised helical ribs extending inward within the chamber. 